In relation to the electrical characteristics (write/erase characteristics) of a semiconductor storage device, the nitrogen concentration profile of the gate insulating film is of great significance. Conventionally, a thermal oxynitridation process is carried out to oxide films. However, this causes deterioration in transistor characteristics, and causes a nitrogen peak to form only at the interface between the oxide film and the substrate, thus it is difficult to form an appropriate nitrogen concentration profile. In other words, when thermal oxynitridation is performed nitrogen peaks are formed at specific locations with a specific concentration, and the formation of these nitrogen peaks is difficult to control.
In Japanese Unexamined Patent Publication No. 2003-60198, a semiconductor device comprising a gate insulating film with a improved nitrogen concentration profile is disclosed.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2003-60198